1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device. More particularly, the present invention relates to a method of manufacturing a metal-oxide-semiconductor (MOS) transistor.
2. Description of the Related Art
At present, metal-oxide-semiconductor (MOS) transistor is one of the major electronic devices of a semiconductor device. In a MOS transistor, spacers on the sidewalls of a gate structure are often fabricated using a dielectric material. To fabricate silicon nitride spacers, silane or dichlorosilane are introduced into a reaction chamber and then heated so that the silane or dichlorosilane reacts with ammonia to form a silicon nitride layer over the substrate to cover the MOS transistor and the gate structure. Thereafter, the silicon nitride layer is etched to form spacers on each side of the gate structure. However, the aforementioned method of forming the silicon nitride layer has a high thermal budget and is rather unsuitable for fabricating the next generation of MOS transistor (having a line width 45/65 nm). Hence, another silicon nitride deposition process that uses hexachlorosilane (HCD) as a precursor material has been developed. Using HCD as a precursor material not only reduces overall thermal budget of the process, but also reduces the amount of hydrogen and produces a layer with a good step coverage and a low loading effect.
Nevertheless, using HCD as a precursor material to deposit a silicon nitride layer followed by etching to form spacers has one major disadvantage. When a pre-cleaning operation using a diluted hydrofluoric acid solution (0.01%) for a metal silicide process is carried out, the etching rate is almost 3 times of a silicon nitride layer formed by reacting silane with ammonia in a conventional rapid thermal chemical vapor deposition (RTCVD) process and almost 6 times of a silicon nitride layer formed by reacting dichlorosilane with ammonia in a conventional deposition process. In other words, the silicon nitride layer formed using HCD as a precursor material may encounter control problems in a pre-cleaning or other wet etching process. If the process is not monitored carefully, the silicon nitride spacers can be over-etched leading to a drop in device performance and/or production yield.